Yes, the OD has it's own drain plug, plus a screen and (on the later ones) magnets to clean.
No reason not to use the dipstick that I can see, in fact I made the swap myself (later gearboxes don't normally have a dipstick, it's only found on TR2 through early 3A). But you can check the plug if you want, to double-check the dipstick. Just make sure it's level, since being tilted either side-to-side or fore/aft will affect the oil level at both dipstick and plug.
I don't pay too much attention to absolute amount, since I'm usually starting with a partially full bottle anyway. But ISTR it takes most of two quarts after draining both gearbox and OD. The level is not super critical anyway, IMO, a little high or low isn't going to hurt anything. If it's way high, it will tend to self-correct by leaking more
If you have the later type drain plug with notches around the edge, I definitely recommend either building or buying a tool to engage more than one notch at a time. I've seen lots of people try to get them off with a hammer and drift (or similar), and the result is often a plug that leaks. Best to have a new gasket on hand too (or suitable O-ring per CJD). Sorry, I forget offhand who sells the tool commercially; here's a shot of the tools I made; with the best one (IMO) in the foreground and the previous version in the background. The old version worked well enough, but was awkward to use as it put your knuckles at risk between the bar and frame if it broke loose suddenly.